1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member and an image forming apparatus provided with the same.
2. Description of the Related Art
An image forming apparatus such as a copying machine and a printer utilizing an electrophotographic method is provided with an electrophotographic photosensitive member for forming electrostatic latent images and toner images. The electrophotographic photosensitive member is required to have an electrophotographic property (i.e. potential characteristic such as a charging characteristic, an optical sensitivity and a residual potential, and an image characteristic such as image density, image resolution, image contrast and image tone) of high quality and stability, as well as durability (against friction, wear, environment and chemical). In order to obtain them, an electrophotographic photosensitive member is suggested to have a conductive body formed with a photosensitive layer including a photoconductive layer and a surface layer.
For the surface layer, various materials and structures have been suggested. An example of the materials includes amorphous silicon materials. Among the amorphous silicon materials, amorphous silicon carbide (a-SiC) containing carbon (C) especially attracts attention as the material of the surface layer that has e.g. high electrical property, high luminous sensitivity, high image property, and high endurance based on high hardness. Further, an electrophotographic photosensitive member provided with a combination of a surface layer made of a-SiC and a photoconductive layer made of amorphous silicon (a-Si) has already been in practical use.
In the photosensitive member containing amorphous silicon, variation in the thickness of the photosensitive layer in the axial direction causes variation in density in the axial direction. Thus, the thickness of the photosensitive layer in the axial direction has been suggested to be substantially constant (i.e. a ratio of the thickness at the end portions to the thickness at the middle portion in the axial direction is not more than ±2%).
JP-A-11-343573 discloses a deposited film forming apparatus provided with a plurality of exhaust holes at an area of a cylindrical adhesion preventing member arranged to surround a cylindrical supporting body and material gas inlet tubes. According to the deposited film forming apparatus, by adjusting the arrangement of the exhaust holes, variation in film thickness of the deposited film in the axial direction of the body can be reduced without complicating the structure of the apparatus and the deposited film forming process.
JP-A-59-213439 discloses a technique for preventing variation in the thickness of the photosensitive layer by providing gas inlet/outlet holes at walls of cylindrical electrodes so that the electrodes serve as gas inlet/outlet member for discharging material gas uniformly into a discharging area.
JP-A-58-30125 discloses a technique for making an electrophotographic photosensitive member without variation in film thickness. The photosensitive member is provided with, separately from a cylindrical electrode, a gas inlet tube for introducing material gas. The gas inlet tube is provided with gas outlet holes whose dimension and arrangement are changed longitudinally of a cylindrical plate, so that material gas is discharged uniformly.
Regarding the surface layer made of a-SIC, the thickness and C content of the surface layer is also studied (see JP-B-01-10069 and JP-A-62-258466, for example).
JP-B-01-10069 discloses a photosensitive member including a surface layer with a thickness of 30 Å-5 μm and carbon content of 40-90 atom %.
JP-A-62-258466 provides a photoreceptive member including a surface layer containing carbon atom with Si atom as the matrix, and concentration of the carbon atom increases as proceeding from a boundary surface with the photosensitive layer to a free surface. In the surface layer of the photoreceptive member, the carbon atom is dispersed at a concentration of 0.5-95 atom %, and thickness of the surface layer is preferably 0.003-30 μm.
However, with the electrophotographic photosensitive member disclosed in the above patent documents, even having a surface layer made of a-SiC, when incorporated in an image forming apparatus utilizing an electrophotographic method and undergoing printing processes, defective images with e.g. flaws and variation in density (resulting in faint images or fogged images) are caused. One of the reasons of such defective images is that the surface layer is ground. For example, toner contains abrasive for grinding a polar surface on the surface layer for preventing the surface layer from adsorbing discharge products generated in corona discharge. Thus, the surface layer is more likely to be ground at the middle portion where images are often printed than at other portions such as the end portions, thereby causing flaws in images.
Therefore, techniques for preventing such flaws in images by changing composition to increase hardness of the surface layer or by increasing the thickness of the surface layer have been suggested and already been in practical use. However, when hardening the surface layer or increasing the thickness, various problems are caused: deterioration in charging ability or increase in residual potential in the photosensitive member; attachment of toner to the surface of the photosensitive member (resulting in a gap between the member and a blade); image deletion; and increase in manufacturing time (film forming time) of the photosensitive member.